This invention relates to a pneumatic radial tire for heavy loads, used for trucks and buses and having a tread pattern based on a rib pattern, and more particularly to a pneumatic radial tire for heavy loads, capable of minimizing the occurrence of partial abrasion thereof and suitably used on a front wheel.
A rib pattern is inferior to a block pattern in the wheel driving characteristics but superior thereto in abrasion resistance. Accordingly, a rib pattern is used in many cases as a tread pattern of a radial tire on a front wheel (steering control wheel) of a heavy load vehicle, such as a truck and a bus. In recent years, the manufacturing of a softer suspension has been developed so as to improve the riding comfort of a truck and a bus, and, with the development of such a suspension, partial abrasion, such as shoulder dropping abrasion, rib punch, wavy abrasion and polygonal abrasion becomes apt to occur on the tread surface of a radial tire of a front wheel.
The pneumatic radial tires heretofore proposed as radial tires in which measures for preventing the occurrence of such partial abrasion are taken include a radial tire provided with narrow grooves in the outer edge portion of shoulder ribs as disclosed in Japanese patent application Kokai publication No. 63-134313, a radial tire provided with cuts in shoulder ribs as disclosed in Japanese patent application Kokai publication No. 61-235205, and a radial tire in which variety is given to the radius of the tread as disclosed in Japanese patent application Kokai publication No. 62-91303.
However, in these techniques, the effects in minimizing the occurrence of partial abrasion are unsatisfactory. Therefore, when wheels are not aligned accurately, various kinds of abrasion is liable to occur on the tread surface, so that the vehicle becomes unable to travel in many cases before the rated lifetime of the tire has terminated.
In view of the above, the inventors of the present invention analyzed in various ways the tires on which partial wear occurred, to discover that the construction of a tire had large influence upon the occurrence of partial abrasion. In a pneumatic radial tire for heavy loads, two to four steel belts are provided as belt layers, and the rigidity of the belt layers is extremely high. Therefore, when a tire having a tread pattern based on a rib pattern as mentioned above is charged with air to be inflated, the shoulders fall from inflection points mainly on the outermost main grooves. When the shoulders thus fall, a difference in diameter between the central portion of a tread and shoulders becomes large, and a dragging frictional force due to this diameter difference is exerted strongly on the shoulders, so that irregular partial wear becomes liable to occur, i.e., the shoulder ribs only are worn earlier.
A pneumatic radial tire for heavy loads has not only a problem of achieving the prevention of partial abrasion thereof but also a problem of improving the draining performance thereof which has close relation with the shape of the grooves formed therein.